1. Field of the Invention
The present invention relates to a lithographic projection apparatus, a mirror, a method of supplying a protective cap layer, a device manufacturing method and a device manufactured thereby.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to a device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission in the case of a transmissive mask, or reflection in the case of a reflective mask, of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-adressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described above, the patterning device can include one or more programmable mirror arrays. More information on mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT Patent Application Publications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is shown in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC's). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction, the “scanning” direction, while synchronously scanning the substrate table parallel or anti-parallel to this direction. In general, the projection system will have a magnification factor M (generally <1), and the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be found, for example, in U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from “Microchip Fabrication: A Practical Guide to Semiconductor Processing,” Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory processes may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796, both incorporated herein by reference.
In the present invention, the projection system will generally consist of an array of mirrors, and the mask will be reflective. The radiation in this case is preferably electromagnetic radiation in the extreme ultraviolet (EUV) range. Typically, the radiation has a wavelength below 50 nm, but preferably below 15 nm, for instance 13.7 or 11 nm. The source of EUV radiation is typically a plasma source, for example a laser-produced plasma or a discharge source.
Optical elements for use in the EUV spectral region are especially sensitive to physical and chemical damage which can significantly reduce their reflectivity and optical quality. Reflectivities at these wavelengths are already low compared to reflectors at longer wavelengths, which is a particular problem since a typical EUV lithographic system may have several mirrors. For instance, a EUV lithographic system may have nine mirrors: two in the illumination optics, six in the imaging optics, plus the reflecting mask. It is therefore evident that even a small decrease of 1-2% in the peak reflectivity of a single mirror will cause a significant light throughput reduction in the optical system.
Reflection losses may be caused by different mechanisms. One of the main causes for reflection losses is a degradation of the reflective surface of the mirror due to oxidation. A further problem is that some sources of EUV radiation, e.g. plasma based sources, are dirty in that they also emit significant quantities of fast ions and other particles which can damage optical elements in the illumination system. Local defects of the reflective surface of the mirror may cause projection errors, resulting in defective chips. Also, carbon present in the low pressure environment surrounding the mirror will precipitate on the reflective surface of the mirror, reducing the reflectivity of the mirror.
In order to protect the mirror against these damaging mechanisms, it is known to provide the reflective surface of the mirror with a protective layer. For example, European Patent Application Publication 1 065 568 A2 describes the use of a cap layer. Many different materials are proposed, such as diamond-like carbon (C), boron nitride (BN), boron carbide (B4C), silicon nitride (Si3N4), silicon carbide (SiC), B, Pd, Ru, Rh, Au, MgF2, LiF, C2F4 and TiN and compounds and alloys thereof.
It will be understood that a thick protective layer gives a good protection against etching by fast ions and other degradation. Also, a certain minimum thickness, depending on the material, is required to obtain a layer that is free of pinholes. However, the protective layer should be as thin as possible, in order to reduce the reflectivity of the mirror as little as possible.
Capped multilayers have been tested that were made from Ru or C, having a typical thickness of approximately 1-3 nm. However, these already showed strong signs of irreversible degradation after approximately 50 hours irradiation under realistic tool conditions. By controlled balancing, or mitigation, of carbon growth and oxidation the life time of an EUV mirror could reach up to 200-2000 hours. Since a desired duration of protection is approximately more than 15,000 hours, for example, 30,000 hours, this is still much too short. Also, there is a high risk of introducing local non-homogeneous degradation, either by local oxidation or by local carbon growth. Such local degradation of the mirror causes local errors in the projected pattern.
A further disadvantage of known cap layers is that the cap layer mixes with the material of the mirror. This is called intermixing and is caused by radiation induced diffusion. Therefore, according to the prior art, an anti-diffusion layer is provided between the cap layer and the reflective surface of the mirror.
It is also known that other components of the lithographic projection apparatus are damaged by, for example, oxidation and/or carbon growth. These other components, such as cables, walls or metal or PTFE constructions with a large area, also need to be protected against damaging mechanisms, in order to increase their lifespan. A further disadvantage of such components is that such components relatively gas out a lot of molecules. Molecules that gas out such components contaminate the system and reduce the vacuum quality.